1. Field of the Invention
The present invention relates to a device and method for the transmission of an analog signal by using a digital type of modem.
Hereinafter in the description, the expression <<digital modem>> designates a modem that receives digital signals at input and gives digital type signals at output. For example, the invention relates to OFDM (Orthogonal Frequency Division Multiplexing), or COFDM (Coherent Orthogonal Frequency Division Multiplexing) type modems or again to serial type modems.
The invention can be applied to audio or speech type analog signals, possibly enciphered signals.
It can be applied also to OFDM type modulation for the high frequencies HF or single-carrier type modulation for the very high frequencies VUHF or again to spread-spectrum modulation, for example by direct sequence or EVF (Evasion Frequency). Single-carrier modulation can also be used in HF and OFDM modulation in VHF.
2. Description of the Prior Art
Various methods are described in the prior art for the radio transmission of a speech or audio signal in total security. Typically, these methods can be classified under two major groups:
1—The analog method, more commonly known as the “scrambler” method.
This type of method has been much used in the HF and VUHF ranges. The associated system consists of a pack, external or internal to the radio unit, which is interposed between the set and the radio unit.
According to one principle, the signal is mixed, typically according to a time-frequency grid pattern made on the basis of a Fourier transform of the input signal. Generally, a synchronization element is added in the form of a signal modulated at a carrier frequency in the audio band. This signal contains the information needed at reception, in order to synchronize the enciphering method. Once the signal has been mixed and is therefore difficult if not impossible to understand, it is transmitted by classic methods chosen according to the range of frequency envisaged, namely SSB (Single Side Band), AM (amplitude modulation) or FM (frequency modulation).
The principle has certain drawbacks. These are especially low resistance and possible interference-related deterioration. Another problem is that the added synchronization signal is not always audible. This method is therefore not very resistant to uncooperative, deciphering attacks, especially because of limits on the number of frequency side bands mixed as a function of the number of points taken in the FFT (Fast Fourier Transformation). By contrast, so long as the synchronization of the cipher is detected at reception, it is possible to descramble the received signal and therefore restore an audible signal even at a low signal-to-noise (S/N) ratio. Since the human ear is a very robust receiver, it restores the auditory signal fairly well even when the signal is highly noise-infested.
2—The Digital Method
There are also known ways of digitizing the speech signal or the audio signal, compressing it by using a vocoder or an audio compressor and then using a digital modem to transmit the bits that are preliminarily enciphered by means of a high-security digital cipher.
This method enables the use of high-quality ciphers. However, this raises certain problems related to bit rate, for example in HF, because there is a limited bit rate available in the conventionally allocated band, 3 kHz, in a digital transmission mode that is resistant to multiple paths (typically, with a spectral efficiency of one 1 bit/Hz/s).
Furthermore, there are limitations when the propagation channel of the signal is highly disturbed. This requires the use of interleaving and of an error-correction code that limits the bit rate, the maximum limit being in the range of 2.4 Kbits/s. The vocoder actually works at low bit rate and is therefore of medium quality. It is even possible to further reduce the bit rate to 800 bits/s, with a low quality vocoder and more robust encoding.
In the latter case, however, there remains the fact that the modem has a point of operation in the Binary Error Rate (BER) adapted to the vocoder and hence a certain signal-to-noise (S/N) ratio that depends on the transmission channel. Below this value of signal-to-noise ratio, the errors are far too numerous for the decoder to be able to correct them, and the signals produced at output of the vocoder are no longer audible, even if the modem is always capable of maintaining the synchronization because it is generally extremely robust.
There are also hierarchical encoding principles enabling the more efficient protection either of the most sensitive bits or of the bits used to reconstitute a minimum quality of the signal transmitted. The systems that implement these principles on the contrary reveal their limits fairly soon. The quality is quite mediocre and/or the signal loss related to the signal-to-noise ratio finally appears fairly quickly.
Similar types of systems exist in the VHF range, with modulations that are rather of the single-carrier type enabling the peak factor (the ratio of peak power to mean power) to be optimized and hence providing for an increase in the efficiency of the output amplifier.